Apple's tool is fine, but the best diff tool I've found for the Mac is (oddly enough) bbdiff which is part of the BBEdit command line tools. Of course, if you don't already own BBEdit it can be a stretch to pay $60 for it just for the diff tool when Apple's is free (Apple's is actually just a GUI for the CLI diff command which you can also use but is nowhere near as nice as having a GUI).